I finally had success after deleting ~/.hplip, then running hp-sendfax, which creates this directory.
Then I followed above steps, sudo chgrp lp ~/.hplip; chmod g+w ~/.hplip
deleted my HP printers, and reinstalled them, using hp-toolbox Advanced options, type: All-in-one, MFP devices only
I'm not sure which steps were necessary, but the fax started working after this
I finally had success after deleting ~/.hplip, then running hp-sendfax, which creates this directory.
Then I followed above steps,
sudo chgrp lp ~/.hplip; chmod g+w ~/.hplip
deleted my HP printers, and reinstalled them, using hp-toolbox
Advanced options, type: All-in-one, MFP devices only
I'm not sure which steps were necessary, but the fax started working after this